I happened to caught a high level Gastly and evolved it into a FB Genger. I agree it's a quick sabotager, but it's also a revive burner. It could be a pain for players with no storage for regular revives (like me).
Personally I think there are way more useful XX pokemon for the old gym system, where to do training efficiently there are many options. After then most pokemons are left into awkward position as they are just dex filler or trade fodder.
I like double fighting Poliwrath, double ice Lapras, but they are still fairly relevant and don't quite qualified for X X pokemon as you defined. Perhaps some non-elite fighting type mons like Primeape, Hitmons could have a use in gym battles.